Argon is a highly inert element used in high-temperature industrial processes, such as steel-making. Argon is also used in various types of metal fabrication processes such as arc welding as well as in the electronics industry, for example in silicon crystals production. Still other uses of argon include medical, scientific, preservation and lighting applications. While argon constitutes only a minor portion of ambient air (i.e. 0.93% by volume), it possesses a relatively high value compared to other major atmospheric constituents (oxygen and nitrogen) which may be recovered from air separation plants. Argon is typically recovered in a cryogenic air separation process as a byproduct of high purity oxygen production. In such processes, an argon rich vapor draw from the lower pressure column is directed to an argon rectification column where crude or product grade argon is recovered overhead.
The availability of low cost natural gas has led to the restart and construction of numerous ammonia production facilities throughout North America. One of the byproducts of ammonia production plants is a tail gas that may be comprised of methane, nitrogen, argon, and hydrogen. This tail gas is often utilized as fuel to fire various reactors within the ammonia production plant. However, if this argon-containing tail gas can be cost-effectively handled and purified, it could be used as an alternative source of argon production.
Ammonia is typically produced through steam methane reforming. In such a process air serves to auto-fire the reaction and to supply nitrogen for the synthesis reaction. In general, the steam methane reforming based process consists of primary steam reforming, secondary ‘auto-thermal’ steam reforming followed by a water-gas shift reaction and carbon dioxide removal process to produce a synthesis gas. The synthesis gas is subsequently methanated and dried to produce a raw nitrogen-hydrogen process gas which is then fed to an ammonia synthesis reaction. In many ammonia production plants, the raw nitrogen-hydrogen process gas is often subjected to a number of purification or additional process steps prior to the ammonia synthesis reaction. In one such purification process, the methane contained in the nitrogen-hydrogen process gas is cryogenically rejected prior to the nitrogen-hydrogen process gas compression. The rejected gas is a tail gas comprising the bulk of the contained methane as well as argon, nitrogen and some hydrogen. This tail gas is often used as a fuel to supply the endothermic heat of reaction to the primary steam reformer.
Argon is present in ammonia tail gas generally contains between about 3% to 6% argon. After hydrogen recovery from the tail gas, the relative concentration of argon increases to between about 12% to 20% argon which makes the argon recovery an economically viable process. In an effort to reduce costs and increase process efficiency, the conventional argon recovery processes from ammonia tail gas are typically integrated with the hydrogen recovery process. The conventional argon recovery processes are relatively complex and involves multiple columns, vaporizers, compressors, and heat exchangers, as described for example in W. H. Isalski, “Separation of Gases” (1989) pages 84-88. Other relatively complex argon recovery systems and process are disclosed in U.S. Pat. Nos. 3,442,613; 5,775,128; 6,620,399; 7,090,816; and 8,307,671.
What is needed, however, is a much simpler and cost-effective system and method for the recovery of argon and nitrogen contained within the tail gas of an ammonia production plant as an alternative source of argon production and/or liquid nitrogen production.